In the field of linguistics, specifically in syntax, phonetic form (abbreviated 'PF'), also known as phonological form or the Articulatory-Perceptual (A-P) system, refers to a certain level of mental representation of a linguistic expression, derived from surface structure, and related to logical form. Phonetic form is the level of representation wherein expressions, or sentences, are assigned a phonetic representation, which is then pronounced by the speaker. Phonetic form takes surface structure as its input, and outputs an audible (or visual, in the case of sign languages), pronounced sentence.